Combat to standard conversion

Hi, tri-man here again.'

My client's Combat motor needs a new cam and we've been discussing options. His riding style and the roads he travels suggests going with a standard cam as I see it.
He has an 850 with a single 34 mm Mikuni from before (also presently in my shop) and he enjoys the easy torque it makes. It has the big port head (RH10? and though I find its performance soft, he thinks it's great.
My idea is to fit the Combat with a standard cam, reduce compression, and sleeve the ports. We're keeping the 32 mm carbs. That would give him the famous Norton bottom and mid range punch, and good throttle response.

Some .040" added to the cylinder base, or head gasket, will reduce CR from 10:1 to around 9:1
Which is preferable, adding to the base or to the head, or a combination?

The Combat's 32 mm ports are IMHO way too big. The Triumph T140 has 28,5 to 29 mm ports and in my view even they are too big unless you want to fit a really high lift cam and go for top end power. So I was planning on sleeving the ports down to 30 mm or less. I'm not going to alter the port shapes in any way, simple tubular sleeves will be employed. What is the standard 750 port size, or more to the point, what would the optimum size be for this application?

There's so much knowledge on here, and any suggestions are most welcome!

Thanks, SR

I would remove it from the pistons. or see if jim comstock comnoz on here has one of the cometic base gaskets. the best option is find an earler head.

I really don't know why people do this. It's a crime. There is nothing wrong with a combat as it is once the bearings have been changed. There is no loss of torque. Perfectly rideable and reliable.

Just buy a stock bike if that's what you want.

Hi batrider,
It's not my bike.
Not having any first hand experience with riding a Combat I can't really argue with your take on this, all I can go with is quotes like "not much torque below 4000" or "needed 5000 rpm to come alive", that sort of thing.
If it was my bike I'd keep it to Combat spec, but it IS a high mileage bike clearly subjected to much abuse over it's life, it could use some (relative) peace and quiet for the rest of it. Besides, the owner would be much happier with a standard tune engine.
A crime? No, neither is it sacrilege. The mods I'm proposing are easily reversible for the next owner, much more so than the sump mod most people do.

So while I appreciate your input I think I'll stick with the plan.

SR

My client's Combat motor needs a new cam and we've been discussing options.
Are you proposing all this work only for your bottom line $?

I'd suggest the KISS method.
First read here: http://atlanticgreen.com/nhth.htm
IMO the simplest most effective and...the ability to restore the bike to an original unbutchered condition is as follows.

1. replace the worn 2S cam with a new 2S combat.
2. upgrade the timing side cam bush to any later style and dump the 06-2600 style.
3. get a very available and relatively cheap (1968-71) standard 750 head 28.5mm port and full .126 squish band.
4. remove insulating washers under intake spring to accommodate 2S cam.
5. leave out cylinder base gasket
6. bolt back on big carbs & manifold
7. original head goes on the shelf for future use

This combo starts easy, runs great use 21t front sprocket for nice cruise and still down to under 20mph in 4th gear or up to 100+
Not expensive, not a lot of work, and reversible
I used this for many years and liked it very much and would not hesitate to do it again.

added: this combo yielded 47RWHP at the 92 rally...the highest stock bike hp at the rally (well almost stock) did I cheat?

Firstly, I'm not proposing any of this work for my own benefit, I'm doing this as a hobby.
Secondly, whether I install a standard or SS cam won't make a dollar in difference to me, so no, please forget the profit angle.
Knocking up a set of sleeves and a base plate wouldn't set the customer back much either.

However, your suggestion makes a lot of sense and I'll look into it. I really appreciate your input.

Regards
SR

Ok got ya.
"client" has somewhat of a commercial connotation ...

You might also want to review:

http://atlanticgreen.com/camsurvey.htm

pn 22729 SS (started as 650SS and 88SS) became stock 20 atlas and became stock 20M3 commando eventually became and called "stock".
Never marked with "SS", the same grind evolved into now (stock) on 20M3S+ (tach drive core) NHT(commando) as 06-1084

2S/combat is the only norton cam marked on the cam SS but is in fact and is named as "2S" 06-3536....not SS

:mrgreen:

Leave the compression at 10 to 1. Fit a PW3 cam from Andover Norton. More power everywhere. Early head as Dave suggests or stick with what he's got.

I would fit a MegaCyle 560-00

Megacycle calls this for an 850 Commando their best "all around" cam with strong midrange and increased top end
I agree as I fitted it to my 850 5 yrs ago

specs here, page 15

http://www.megacyclecams.com/catalog/ca ... imized.pdf

drop the compression a little given the type of riding your client has expressed

he is certainly not looking for high rpm high compression performance especially with his choice of a single Mikuni

I think there's a little confusion here. The engine tri-man is looking for advice for is a 750 Combat. He just mentioned that his client also has an 850 with a single mikuni. That's not the engine he's discussing mods to.

Ken

i detuned a combat for a mate of mine ,i made and fitted a 1mm plate under the barrel ,also changed the crankcases to an 850 set because i had a spare set so didnt need to modify the scavenge on the combat ones,fitted longer pushrods to keep the geometry right on the rocker arms ,and fitted a single mikuni it had loads of grunt low down ,later on it went back to twin amals that was 15 years ago and the bike is still running sweet to this day .

for a street engine I would NOT have a compression ratio over 9 - 1 as fuel can become an issue . as I and dyno dave stated I would find a non combat head as it is a known issue with them being over ported. the non combat head is a win win as it will put the compression in a better ratio and the smaller ports are also better. I am also not a fan of a spacer plate under the cylinder. as to a camshaft I have used the megacycle 56-000 and the web cam 12 grind from jim and find the 12 to be a MUCH better street cam wit a VERY broad spread of power and the 56-000 a better hot street cam in bigger motors as it is even hotter than the combat 2 S.

Would not going to oversized intake valves improve the torque performance of the combat with the 32MM intake bore? Would there be any overlap valve clash issues with the 2S cam?
Cheers,
Thomas

keep your eye on the prize?...."That would give him the famous Norton bottom and mid range punch"

He wants a stout touring engine with guts= IMO dual carbs... no single mikuni

mega cycle 56000-not my choice
Web12 OK sort of

PW3... still exhaust opens to early...chilled iron... easy to break in my own experience and also according to reports, I would not use one even if given for FREE. profile is otherwise OK.

Combat even better.... ramps could be better..... OEM for his motor
megacycle 560-NR best non stock cam... IMO... for this guys expressed use...
Stock 06-1084 best

I'm happy with the response to this thread, lots of stuff to ponder. Thanks to all of you.

A little back ground info:
While the bike's owner ("client" perhaps sounding a bit pretentious...) is a Norton fan, his experience is mainly with other (Jap) makes. His 850 Commando has never given him the reliability or user friendliness he was used to and it leaked really bad, but it gave him a tantalizing taste of what a Commando can deliver, and he wants more of it! Low and mid range punch that is, he rarely exceeds 6000 rpm and though I feel the single Mikuni on his 850 stifles the performance, he's happy with it. Which is why I feel a "retune" to enhance low and mid range torque to be in his best interest on the 750.
The Combat wouldn't be MY choice for HIS needs, but he bought it at a good price, the PO has thrown a lot of money at it and it looks good. However, it has led a hard life and I thought that a standard tune would help keep things together for a longer time.
A standard cam normally gives higher cylinder pressure at cranking speeds, which is why I'd like to reduce the CR. Smaller ports would normally enhance low and mid range response too, which is why I talked about sleeving the ports.
Changing to an early head sounds like a plan, if I could find a decent one.

However, batrider and others seems to feel that the Combat tune doesn't seem to lose much, if any low/mid range at all, and as the owner won't thrash it anyway, maybe we should simply stick with the original tune?
Besides, the 850 is good now, so he's already got his torque cravings satisfied.

Hmmm...
SR

Without wanting to hijack this thread, many years ago in my youthful search for more speed I had Mick Hemmings open up my 1970 750 ports to 32mm and more recently I've fitted a PW3 cam and run a pair of 32mm PWK carbs from Jim Schmidt. Performance is ok but not as sharp as I seem to remember in the early days. (particularly at less than 4000rpm) I have recently come across a standard head with 30mm ports and still have my old 30mm carbs and standard cam. I am seriously tempted to return it all to as near standard as possible and see if I can improve things. Any thoughts on this plan? (My original head has standard valve sizes)

gripper said:

I have recently come across a standard head with 30mm ports

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It must be a domestic market head only... we never got any factory 30mm port 750 heads in north america :shock:
...only RH1 28.5mm port and RH3 RH5 RH6 32mm port... not carb

Good point, I'll measure it properly.

I've been doing a bit of measuring myself today, the Combat head in my care has ports that measures 32 mm at the manifold flange and tapers off to 28,5 mm at the valve guide. I should have done these measurements before my first post, I'm ashamed I didn't. I assumed it was 32 mm all the way because "someone told me so", assumption is the mother of all FU's.
This port shape and size should work well enough for the owner's requirements I think, especially since guides, valves and seats are in good shape, apparently fitted by someone who knew and cared.

Which leaves me with the cam and compression considerations.

I've had so many good recommendations from you all, but by now I think it boils down to this: standard cam with or without a compression reduction, or the same with the 2S cam. The owner has a say in this, he'd prefer to stick with Norton bits.

Which again leads to the question I should have asked in the first place:
"Would a well set up Combat engine be suitable for a rider that revels in solid low and mid range punch rather than top-end, or what kind of mods would you suggest to bend it in that direction?"

Dave? hobot? Others?

here is one way to go with it. use the combat head and cut the piston tops .035 and no base gasket, stay with the combat cam ( I do not like the english ground replacement cams ) but DO NOT use the tab washer or at least remove the tab or change the timing side bush to the flanged type. super blend mains. DO NOT butcher the cases for the oil pickup mod but move it to the rear of the sump in another fashion. DO NOT relocate the breather , instead use the CNW read breather @ the stock location. gear it with a 21 tooth countershaft sprocket. this combo will make a good runner but still will need 92 octain fuel but COULD sneak by with a SMALL mix of 89 in a have to case. this combo is still happyist over 3500 RPM .

second option is to do it as listed BUT use a standard cam and add the base gasket BUT get the one sold be comnoz to drop the cylinder pressure a little more and get some more low end. will still need premium fuel.

tri-man said:

Which again leads to the question I should have asked in the first place:
"Would a well set up Combat engine be suitable for a rider that revels in solid low and mid range punch rather than top-end, or what kind of mods would you suggest to bend it in that direction?"

Dave? hobot? Others?

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